Cobalt on alumina catalysts

ABSTRACT

A cobalt on transition alumina catalyst, containing between 3 and 40% by weight of cobalt has a cobalt surface area above 30 m 2  / g  of cobalt, preferably above 50 m 2  / g  of cobalt, most preferably above 80 m 2  / g  of cobalt.

This application is the national phase of international applicationPCT/EP95/02836, filed Jul. 17, 1995 which designated the U.S.

This application is the national phase of international applicationPCT/EP95/02836, filed Jul. 17, 1995 which designated the U.S.

FIELD OF THE INVENTION

The present invention relates to cobalt on alumina catalysts and to aprocess for making the same.

BACKGROUND OF THE INVENTION

In catalysts, cobalt is normally fixed on a carrier such as silica,aluminium silicate, alumina.

In these catalysts, the useful cobalt atoms are those which are exposedat the surface of the cobalt particles. The cobalt atoms which are notexposed (i.e not at the surface) will not participate in catalyticreaction.

Cobalt is an expensive metal and, in order to optimize its use as acatalyst, it is essential to increase as much as possible the (number ofexposed cobalt atoms/ total number of cobalt atoms) ratio of thecatalyst which, in turn, increases the cobalt surface area per gram ofcobalt.

It is known from EP-A-13,275 to produce a supported coprecipitatedcobalt-silica catalyst wherein a reaction mixture of cobalt ions,silicate ions and solid porous carrier particles is prepared and allowedto form a coprecipitate of cobalt and silicate ions onto the solidporous support particles. The obtained cobalt-silica catalyst has a BETtotal surface area ranging from 150 to 350 m² /g and a cobalt surfacearea ranging from 5 to 20 m² /g of cobalt.

It is also known from U.S. Pat. No. 4,591,579 to provide a process forthe preparation of a transition metal-silicate catalyst in which aninsoluble, basic compound of a transition metal (e.g. cobalt, nickel orcopper) is precipitated with an alkaline precipitation agent from anaqueous solution of such a metal salt after which this compound isreacted with a silicate solution. In example 5, it is described such acatalyst wherein the cobalt surface area is 8.9 m² /g of catalyst.

In `Stoichiometries of H₂ and CO Adsorptions on cobalt`--Journal ofCatalysis 85, page 63-77 (1984)--are disclosed on page 67, table 1,cobalt catalysts on different carriers. From the total maximum H₂uptake, it is possible to calculate the cobalt surface area per gram ofcatalyst and the cobalt surface area per gram of cobalt. It can be seenthat, for cobalt on silica catalysts the cobalt surface area per gram ofcobalt ranges between 6 and 65 m² /g whereas for cobalt on transitionalumina catalysts the cobalt surface area per gram of cobalt rangesbetween 15 and 26 m² /g.

Thus, cobalt catalysts with a high cobalt surface area per gram ofcobalt exist for cobalt on silica catalysts (and also for cobalt oncarbon catalysts), but don't exist for cobalt on transition aluminacatalysts.

Nevertheless, cobalt upon transition alumina catalysts present somedistinct advantages towards other cobalt catalysts.

First of all, a cobalt on transition alumina catalyst is easier to shapeby extrusion than a cobalt on silica catalyst and the mechanicalstrength of the resulting catalyst is higher.

In reactions where water is present (e.g. methanation, Fisher Tropsch),silica can be unstable. Alumina, however, is much more stable under suchconditions.

There is therefore a need for a cobalt on transition alumina catalystwith a cobalt surface area per gram of cobalt higher than previouslyobtained.

Its is a first goal of the present invention to provide a cobalt ontransition alumina catalyst with a cobalt surface area per gram ofcobalt higher than previously.

It is a second goal of the present invention to provide a process formanufacturing such catalyst.

Tests and Definitions

i) cobalt surface area

Approximately 0.5 g of sample is used for the analysis. The weight usedto calculate the metallic surface area is that obtained afterpretreatment. During this pretreatment the sample is degassed and driedunder vacuum at 120° C. The pretreated sample is then reduced. Sample isheated to 425° C. at a rate of 3° C./min whilst hydrogen gas is passedthrough the sample at a flow rate of 250 ml/min. Still with the samehydrogen flow the sample is maintained at 425° C. for 18 hours. Undervacuum the sample is heated up to 450° C. over a 10 min time period. Thesample is maintained at 450° C. under vacuum for 2 hours.

The chemisorption analysis is carried out at 150° C. using pure hydrogengas. An automatic analysis program is used to measure a full isotherm upto 800 mmHg pressure of hydrogen.

The method is to extrapolate the straight-line portion of thechemisorption isotherm between 300 and 800 mmHg to zero pressure tocalculate the volume of gas chemisorbed (V).

Metallic surface areas were calculated in all cases using the followingequation, ##EQU1## Where V=uptake of hydrogen in ml/g SF=Stoichiometryfactor (assumed to be 2 for H₂ chemisorption on Co) A=area occupied byone atom of cobalt (assumed to be 0.0662 nm²)

This equation is disclosed by Micromeretics in Operators Manual for ASAP2000 Chemi System V 1.00, Appendix C, Part No. 200-42808-01, 18 January1991.

ii) Transition alumina

Transition aluminas are defined in "Ullmans Encyklopaedie dertechnischen Chemie", 4., neubearbeitete und erweiterte Auflage, Band 7(1974), pp.298-299.

The document divides transition aluminas in several categories:

-gamma-group

Included in the gamma-group are, apart from gamma-Al203, alllow-temperature forms such as eta-A1203 and chi-A1203. They are formedon calcination of aluminiumhydroxides at 400°-750° C.

The specific surface area of gamma-group forms of aluminas is in therange of 150-400 m² /g.

delta-group

The delta group of aluminas includes all high-temperature forms, e.g.delta-, theta- and chi-Al203. The delta group aluminas are formed onheating gamma-group aluminas at approximately 800° C. or higher.

The specific surface area of delta-group forms of aluminas is in therange of 50-150 m² /g.

General Description of the Invention

It is a first object of the present invention to provide a cobalt, ontransition alumina support, catalyst, containing between 3 and 40% byweight-of cobalt wherein the cobalt surface area is above 30 m² /g ofcobalt, preferably above 40 m² /g, more preferably above 50 m² /g ofcobalt, even more preferably above 80 m² /g of cobalt.

Preferably, the transition alumina support is a gamma alumina or a thetaalumina, more preferably a theta alumina.

Preferably, the cobalt catalyst contains 5% to 20% by weight of cobalt,more preferably 10 to 20 % by weight.

It is a second object of the present invention to provide a process formanufacturing a cobalt, on transition alumina support, catalyst,containing between 3 and 40% by weight of cobalt, the cobalt surfacearea being above 30 m² /g of cobalt, wherein a slurry of transitionalumina in an aqueous solution of cobalt ammine carbonate is heated to atemperature of 60° C. to 110° C., in order to allow cobalthydroxycarbonate to precipitate, the resulting product being then driedand calcined. Optionally, the calcined product may be further reduced.

Preferably, the transition alumina support is a gamma alumina or a thetaalumina, more preferably a theta alumina.

It is a third object of the present invention to provide a process formanufacturing a cobalt, on transition alumina support, catalyst,containing between 3 and 40% by weight of cobalt, the cobalt surfacearea being above 30 m² /g of cobalt, wherein, transition aluminaparticles are saturated with an aqueous solution of cobalt amminecarbonate, the excess solution being removed by filtration, theresulting product being heated to a temperature of 60° C. to 110° C. ,in order to allow cobalt hydroxycarbonate to precipitate, the resultingproduct being then dried and calcined.

Preferably, the transition alumina support is a gamma alumina or a thetaalumina, more preferably a theta alumina.

Successive impregnation and precipitation steps may be applied toincrease the cobalt content, the deposited cobalt hydroxycarbonate beingconverted into cobalt oxides during a calcination treatment at atemperature of 200° to 600° C.

The product can then be activated with hydrogen gas at temperaturesbetween 200° and 600° C. preferably between 350° and 550° C. and thenoptionally passivated

Specific Description of the Invention

The present invention will be further described in the followingexamples.

EXAMPLE 1

i) Preparation of the impregnation solution.

Weigh out 1764.0 g of a 35% ammonium hydroxide solution (specificgravity 0.88) (obtainable from BDH) and add 73.5 g of demineralisedwater. Add 312.5 g of ammonium carbonate (obtainable from Merck) andstart stirring. Heat gently to 35° C. to assist dissolving the powder.When fully dissolved, add slowly 350 g of basic cobalt carbonate(obtainable from Merck and containing 47-55 wt % Co). Continue stirringfor about 2 hours. Filter through Buchner funnel.

ii) Impregnation of 8-Al₂ O₃ support

Weigh out 100 g of theta alumina into a beaker and add 0.5 1 of theimpregnation solution. After 10 minutes put the impregnated extrudateson a Buchner filter to drain excess liquid. Then dry the product for 1hour at room temperature then 1 hour at 80° C. and finally overnight(16h) at 120° C. Eventually, the dried product is calcined in an airflow at 350° C. for 2 hours using a rotary calciner.

EXAMPLE 2

The dried product obtained in Example 1, after impregnation and beforecalcination is impregnated once again as disclosed in Example 1, underii) and dried as disclosed in Example 1 under ii).

Eventually, the dried product is calcined in an air flow at 350° C. for2 hours using a rotary calciner.

EXAMPLE 3

The dried product obtained in Example 2, after impregnation and beforecalcination is impregnated once again as disclosed in Example 1, underii) and dried as disclosed in Example 1 under ii).

Eventually, the dried product is calcined in an air flow at 350° C. for2 hours using a rotary calciner.

EXAMPLE 4

The dried product obtained in Example 3, after impregnation and beforecalcination is impregnated once again as disclosed in Example 1, underii) and dried as disclosed in Example 1 under ii). Eventually, the driedproduct is calcined in an air flow at 350° C. for 2 hours using a rotarycalciner.

The products obtained in Examples 1 to 4 were then analysed and theresults are as follows:

    ______________________________________                Ex. 1                     Ex. 2     Ex. 3  Ex. 4    ______________________________________    Co (w/w %)    5.9    9.5       11.7 13.2    Surface area (m.sup.2 /g                  55.9   91.6      98.3 101.5    of cobalt)    Surface area (m.sup.2 /g                  3.3    8.7       11.5 13.4    of catalyst)    ______________________________________

It can be seen that cobalt on alumina support catalysts with a very highcobalt surface area (per gram of cobalt) can be obtained which enablesto have cobalt surface areas (per gram of catalyst) comparable withthose obtained in the prior art but with a much lower cobalt content.

A catalyst according to example 3, after having been activated, wastested for its catalytic activity in the hydrogenation of aromatics,using toluene as a probe.

Reaction conditions: Pressure: 40 bar H₂ LHSV: 3 Hydrogen flow: 15ml/min Temperature: 200° C. Catalyst loading: 6 ml (i.e 5 g)

The conversion of toluene into hydrogenated products was 62% provingthat the product of the present invention can be used as anhydrogenation catalyst.

We claim:
 1. A catalyst, comprising cobalt on a support of transitionalumina, having a cobalt content between 3 and 40% by weight, and which,when reduced with hydrogen at 425° C., has a cobalt metal surface areaabove 30 m² per g of cobalt.
 2. A catalyst according to claim 1, whereinsaid cobalt metal surface area is above 40 m² per g of cobalt.
 3. Acatalyst according to claim 1, wherein said cobalt metal surface area isabove 50 m² per g of cobalt.
 4. A catalyst according to claim 1, whereinsaid cobalt metal surface area is above 80 m² per g of cobalt.
 5. Acatalyst according to claim 1, wherein the support is theta alumina. 6.A catalyst according to claim 1, wherein said catalyst has a cobaltcontent between 5 and 20% by weight.
 7. A catalyst according to claim 1,wherein said catalyst is in the reduced state.
 8. A process formanufacturing a catalyst of cobalt on a support of transition aluminaand having a cobalt content between 3 and 40% by weight, and, whenreduced with hydrogen at 425° C., has a cobalt metal surface area above30 m² per g of cobalt, said process comprising heating a mixture oftransition alumina and an aqueous solution of cobalt amine carbonate toa temperature of 60° C. to 110° C. in order to allow cobalthydroxycarbonate to precipitate, drying and calcining the resultingproduct at a temperature below 60° C.
 9. A process according to claim 8,wherein said transition alumina is a theta alumina.
 10. A processaccording to claim 8, further comprising saturating transition aluminaparticles with an aqueous solution of cobalt amine carbonate, andremoving the excess of the solution before heating the resulting productto a temperature of 60° C. to 110° C.
 11. A process according to claim10, wherein said transition alumina is a theta alumina.
 12. A processaccording to claim 8, wherein the transition alumina is in the form ofextrudates.
 13. A process according to claim 8, further comprising thestep of reducing the calcined product.